Gas lasers and their operation are well known. Such gas lasers are described in U.S. Pat. Nos. 4,674,092, 4,736,379 and 4,378,600 which are incorporated herein by reference. In the operation of a gas laser an electric voltage and current is maintained as appropriate across a cathode and an anode within a gas discharge tube. It is important that the proper amount of gas pressure be maintained to ensure optimal performance of the laser. More importantly, if gas pressure becomes too low, the laser discharge will become unstable and the anode will overheat with possible catastrophic results.
In commercial ion lasers, gas pressure is frequently measured indirectly. For example, gas pressure can be monitored by measuring variations in the voltage between the anode and the cathode. As the gas pressure is reduced, the voltage will change in a predictable manner allowing the operator or an automatic gas refill system to add an appropriate amount of gas to return the gas pressure to its desired operating level. Unfortunately, other factors such as cathode degradation and bore erosion may also affect the cathode to anode voltage in a manner that resembles a decrease in gas pressure. In the event the voltage measured changes due to one of these other factors, gas will be erroneously added resulting in having too high a gas pressure within the discharge tube for optimal laser performance.